This invention relates generally to line receivers and particularly to line receivers having desirable dynamic range and filtering capabilities in a single amplifier stage.
DSL (digital subscriber line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephones lines. xDSL refers to different variations of DSL such as ADSL (asymmetric DSL), G.Lite DSL (ITU-T standard G-992.2), HDASL (high bit-rate DSL) and RADSL (rate-adaptive DSL).
DSL modems are typically installed in pairs, with one of the modems installed in a home (customer""s premises) and the other in the telephone company""s central office servicing that home. The pair of xDSL modems are connected to the opposite ends of the same twisted-pair transmission line.
Referring to FIG. 1 a conventional xDSL communication system 100 comprises a CO (central office) 101. The CO 101 has a plurality of xDSL modems 102 (only one shown). The xDSL modem 102 has a D/A (digital to analog) converter 104. An output of the D/A converter 104 is connected 105 to an input of an xDSL driver 106. An output of the xDSL driver 106 is connected 107 to a 4-wire input of a hybrid 108. A 4-wire output of the hybrid 108 is connected 109 to an input of an xDSL receiver 110. An output of the xDSL receiver 110 is connected 111 to the input of an A/D (analog to digital) converter 112. An AGC (automatic gain control) output of the A/D converter 112 is connected 113 to an AGC input of the xDSL receiver 110. A 2-wire port of the hybrid 108 is connected to a transmission line 114, such as copper twisted pair.
The xDSL communication system 100 also comprises CPE (customer premises equipment) 126. The CPE 126 has an xDSL modem 122 having a D/A converter 124. An output of the D/A converter 124 is connected 125 to an input of an xDSL driver 126. An output of the xDSL driver 126 is connected 127 to a 4-wire input of a hybrid 128. A 4-wire output of the hybrid 128 is connected 129 to an input of an xDSL receiver 130. An output of the xDSL receiver 130 is connected 131 to an input of an A/D converter 132. An AGC output of the A/D converter 132 is connected 133 to an AGC input of the xDSL receiver 130. The 2-wire port of the hybrid 128 is connected to the transmission line 114.
Since an xDSL modem operates at frequencies higher than the voice-band frequencies, an xDSL modem may operate simultaneously with a voice-band modem or a telephone conversation. Referring to FIG. 2, there is shown an example of a frequency spectrum plan 200 for a G.Lite DSL system on the transmission line 114 of FIG. 1. The frequency range from 0.3 to 4 kHz 202 is occupied by conventional voice communications. The frequency range from 30 to 120 kHz 204 is occupied by upstream (CPE 126 to CO 101) data transmission. The frequency range from 150 kHz to approximately 500 kHz 206 is occupied by downstream (CO 101 to CPE 126) data transmission. The upper frequency limit of the downstream data transmission is determined by the length and quality of the transmission line 114.
A problem in xDSL communications systems is that the hybrid 108 in the CO 101 does not provide adequate isolation between the xDSL driver 106 in the CO 101 and the xDSL receiver 110 in the CO 101. Similarly, the hybrid 128 in the CPE 126 does not provide adequate isolation between the xDSL driver 126 in the CPE 126 and the xDSL receiver 130 in the CPE 126. This lack of isolation causes unwanted noise, distortion and saturation.
One possible solution is to use a Salen and Key circuit for the xDSL receiver 110 and the xDSL receiver 130 of FIG. 1. Referring to FIG. 3, there is shown a conventional implementation of a Salen and Key circuit 300 that comprises a first impedance 302 (Z1) having a first terminal connected to an input node 320 and a second terminal connected to a first junction node 322; a second impedance 304 (Z2) having a first terminal connected to the first junction node 322 and a second terminal connected to a second junction node 324; a third impedance 306 (Z3) having a first terminal connected to the second junction node 324 and a second terminal connected to a ground reference 314; a fourth impedance 308 (Z4) having a first terminal connected to the first junction node 322 and a second terminal connected to an output node 326; and an amplifier 310 that is preferably an operational amplifier (opamp) having a non-inverting input 310A connected to the second junction node 324, an output 310C connected to the output node 326 and an inverting input 310B connected to the output 310C. The input node 320 is connectable to a voltage source (shown in dotted outline at 312) such as a 4-wire output of a hybrid. The output node 326 is connectable to a load (not shown) such as an input of an A/D converter.
In the xDSL receiver 110 in the CO 101 the downstream data transmission 206 must be rejected and the upstream data transmission 204 be passed. It is therefore advantageous that the xDSL receiver 110 in the CO 101 be a low-pass filter. To this end, referring to FIG. 3, the first impedance 302 and second impedance 304 are implemented as resistors while the third impedance 306 and fourth impedance 308 are implemented as capacitors. The values of the four impedances 302, 304, 306,308 are chosen such that the cut-off frequency of the low-pass filter is between the frequency of the upstream data transmission and the frequency of the downstream data transmission. For example, in the case of G.Lite DSL, the cut-off frequency would be between 120 kHz and 150 kHz.
Conversely, in the xDSL receiver 130 in the CPE 126 the upstream data transmission 204 must be rejected and the downstream data transmission 206 be passed. It is therefore advantageous that the xDSL receiver 130 in the CPE 126 be a high-pass filter. To this end, referring to FIG. 3, the first impedance 302 and second impedance 304 are implemented as capacitors while the third impedance 306 and fourth impedance 308 are implemented as resistors. The values of the four impedances 302, 304, 306, 308 are chosen such that the cut-off frequency of the high-pass filter is between the frequency of the upstream data transmission and the frequency of the downstream data transmission. For example, in the case of G.Lite DSL, the cut-off frequency would be between 120 kHz and 150 kHz.
A problem with the conventional Salen and Key circuit of FIG. 3 is that the pass-band gain is fixed. As well, the dynamic range is limited by the presence of leakage from the xDSL drivers 106, 126 through the hybrids 108, 128.
Two possible solutions are shown in FIGS. 4A and 4B. In FIG. 4A the topology is the same as FIG. 3 except that an adjustable gain amplifier 404 having an input 404A and an output 404B is inserted such that the output 404B is connected to the input node 320. However, there is no protection against large interfering signals in the adjustable gain amplifier 404. In FIG. 4B the topology is the same as FIG. 3 except that an adjustable gain amplifier 406 having an input 406A and an output 406B is inserted such that the input 406A is connected to the output node 326. However, the circuits of FIGS. 4A and 4B have the disadvantages of high noise, high power, distortion and complexity.
Thus there is a need in the industry to provide an xDSL receiver with improved dynamic range. Furthermore, it would be advantageous to provide an xDSL receiver that would also have filtering capabilities, low power consumption, less complexity and good noise figure.
The invention may be summarised according to a first broad aspect as a line receiver having an input and an output, equipped with an amplifier that is preferably an operational amplifier, a network of four impedances and a variable gain amplifier. The first impedance is connected from the input of the line receiver to a first junction node, the second impedance is connected from the first junction node to a second junction node, the third impedance is connected from the second junction node to a ground reference and the fourth impedance is connected from the first junction node to a third junction node. The amplifier has a non-inverting input connected to the second junction node, an inverting input connected to the third junction node and an output connected to the output of the line receiver. The variable gain amplifier has an input connected to the output of the line receiver, an output connected to the third junction node and an AGC input.
In accordance with this first broad aspect of the invention the variable gain amplifier has a gain of   1  K
where K greater than 0 and is set by the AGC input. The network of four impedances can be arranged to realise a low-pass or a high-pass filter.
The invention may be summarised according to a second broad aspect as a line receiver having an input and an output, equipped with an amplifier that is preferably an operational amplifier, a network of four impedances and a variable gain amplifier. The first impedance is connected from the input of the line receiver to a first junction node, the second impedance is connected from the first junction node to a second junction node, the third impedance is connected from the second junction node to a ground reference, the fourth impedance is connected from the first junction node to a third junction node. The amplifier has an inverting input connected to the second junction node, an non-inverting input connected to the third junction node and an output connected to the output of the line receiver. The variable gain amplifier has an input connected to the output of the receiver, an output connected to the third junction node and an AGC input.
In accordance with this second broad aspect of the invention the variable gain amplifier has a gain of   -      1    K  
where K greater than 0 and is set by the AGC input. The network of four impedances can be arranged to realise a low-pass or a high-pass filter.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of the specific embodiments of the invention in conjunction with the accompanying figures.